In recent years, various functionalities possessed by peptides have been revealed. For example, it has been reported that Gly-Tyr and Ser-Tyr have an angiotensin converting enzyme inhibitory activity and a hypotensive activity (e.g., see Non-patent Reference 1). Additionally, anti-oxidation, anticancer, inhibition of blood pressure increase, lowering of blood cholesterol, opioid activity and the like have also been reported for various peptides (e.g., see Non-patent Reference 2). Accordingly, various physiological effects which contribute to human health can be expected by ingesting peptides.
As the production method of peptides, a method for chemical synthesis and a method for hydrolyzing a protein-containing material with a protease preparation are broadly used as conventionally known methods. However, the former case is unsuitable for oral ingestion because it uses chemicals and the latter case has a problem in that the production cost becomes high and oral ingestion cannot be made easily because of the strong bitterness of peptides.
As the means for solving the bitterness of peptides, several techniques have so far been devised. For example, there have been disclosed a method for adding cyclodextrins (e.g., see Patent Reference 1), a method for adding oligosaccharides (e.g., see Patent Reference 2), a method for adding organic acids and saccharides (e.g., see Patent Reference 3), a method for adding phospholipids (e.g., see Patent Reference 4), a method for contact treating with synthetic absorbing resins (e.g., see Patent Reference 5), and the like. However, it is the present situation that these cannot be said sufficient in terms of their effects, such as generation of a new foul taste due to masking agents, considerable increase in the production cost, and the like.
By the way, soy sauce is a seasoning which has been used from early times, and its rich deliciousness is created by the hydrolysis of soybean and wheat with the enzymes produced by Aspergillus. The deliciousness is produced by the solubilization of material proteins to be peptides through the activity of proteases and additional degradation of the peptides into amino acids including glutamic acid by the activity of peptidases. Accordingly, the technical development of soy sauce over many years has been carried out by putting the primary object in strengthening its deliciousness by degrading the proteins and peptides into amino acids to the utmost.
In the brewing of soy sauce, it is considered that peptides are remained in soy sauce when they are not degraded by peptidase after release of the peptides by protease. Accordingly, it is expected to have various profitable physiological activities as described in the above. However, since it is considerably difficult to specifically control the peptidase activity in the general preparing step, such a soy sauce or fermented seasoning has not so far been invented.